Carbide tipped saws have for a long time been used for cutting industrial non-ferrous metals such as aluminum, brass and copper, and it is well known that for such work they have marked advantages over saws with high-speed steel teeth. Although a carbide tipped saw is higher in cost than a high-speed steel saw, it usually cuts faster and almost invariably operates for much longer periods of time between sharpenings. Therefore, taking into account both first cost and the cost of sharpenings through the life of the saw (but disregarding other considerations for the moment) a carbide tipped saw can perform far more cutting per dollar than a high-speed steel saw.
Notwithstanding this important advantage, carbide tipped saws have not replaced high-speed steel saws for non-ferrous metal cutting, and in fact the great majority of saws now used for cutting non-ferrous metals have high-speed steel teeth.
The reason for this peculiar situation is that heretofore conventional carbide tipped saws for industrial non-ferrous metals could not be used on sawing machines designed for operation with saws having high-speed steel teeth, and most of the sawing machines now in use were designed for saws of the latter type. A heretofore conventional carbide tipped saw for non-ferrous metals has had to be driven at surface speeds (i.e., peripheral speeds) eight to ten times as fast as those needed for a comparable high-speed steel saw. To develop such high speeds, the sawing machine has to have a substantially high horsepower drive motor as well as expensive bearings, transmission elements and the like, and therefore its cost is on the order of three times that of a slower rotating machine for a high-speed steel saw.
Naturally there were a good many cases where the high first cost of the machine was more than offset by the savings to be realized with the use of a carbide tipped tool, but more often production volume has not justified investment in a machine intended for conventional carbide tipped saws, and it has been economical to accept the disadvantages of the more expensive high-speed steel saws and operate with the less expensive machine. As a result, the great majority of sawing machines now in use are slow-speed machines intended for highspeed steel saws.
It has been evident for a long time that an ideal situation would be created by the development of a carbide tipped saw that could satisfactorily cut industrial non-ferrous metals when installed on a machine designed for high-speed steel saws. From the very fact that a heretofore conventional carbide tipped saw was always driven at high surface speeds when cutting non-ferrous metals, there was no reason to believe that such a saw could be used satisfactorily on a low-speed machine, but the mere possibility of achieving some net gain, even at the cost of poor cutting quality, encouraged experiment. There may well have been other tests of this kind that were not known to the applicants, but the one that they know of was an attempt to cut through 8-inch diameter aluminum bars. Several carbide tipped saw designs were tried, all of which failed. In the nearest approach to success, the cuts were so rough that the workpiece had a torn appearance and the saw seemed to be smashing its way through the workpiece rather than cutting. This poor cutting quality was not offset by cutting economy. Instead, the tooth tips and blade body quickly became galled with aluminum, and the maximum performance achieved was 179 cuts, totalling 9,000 sq. in., feeding 30 inches minimum at 85 rpm.
These results raise the question of why a high-speed steel saw is very satisfactory for cutting non-ferrous metals when operating at a speed in the range of 700 to 1200 S.F.M. whereas a conventional carbide tipped saw is hopeless at such speeds although capable of superior performance at surface speeds six to ten times as high. In part the present invention resides in the discovery of the answer to this question, and in part it resides in discovering a remedy for the condition discovered.
When non-ferrous metal is being cut, the cutting tooth must have a positive rake, that is, the front surface of the tooth should have some radial inward inclination so that the tooth is slightly hook-shaped. This positive rake is needed in order to force the removed metal to curl down into the tooth gullet instead of breaking off at the cutting edge and clogging the kerf. Positive rake is needed on the teeth of both high-speed steel saws and carbide tipped saws.
It has been found that when a carbide tipped saw with positively raked teeth is working on non-ferrous metal while rotating at a relatively low speed, the teeth tend to wander from side to side in the kerf. In a carbide tipped saw each carbide insert is slightly wider than the blade body and is ground to have its side surfaces slightly convergent rearwardly from its front surface. The insert therefore has no more than line contact with the side surfaces of the kerf, along the side edges of its front face. Those edges cannot provide enough support to stabilize the tooth against lateral excursions, and to make matters worse they act as cutting edges that remove metal from the side surfaces of the kerf and thereby afford more freedom for such excursions. The deflection forces upon the cutting tooth are carried into the blade body and tend to set the saw as a whole into a vigorous lateral vibration that results in the rough and crooked cuts, fracturing of the carbide tips and aluminum galling that were observed in the above described experiment.
The teeth of a high-speed steel saw do not have side clearances, their side surfaces being coplanar with the saw body. Therefore, even though high-speed steel cutting teeth may be subjected to the same lateral forces that act upon a carbide tipped tooth, the side surfaces of the kerf effectively confine the teeth and the saw body against lateral excursions.
In the case of a conventional carbide tipped saw cutting non-ferrous metal while rotating at the high speed for which it is intended, there is an altogether different explanation for the relatively smooth cutting operation of the tool. At high surface speeds a cutting tooth exerts a relatively high forward force upon the metal of the workpiece whereby a curling chip is more or less peeled off of it without generating sideward deflecting forces that tend to set the saw into lateral vibration.
It will be apparent from this explanation that neither the prior art teachings relating to high-speed steel saws nor those relating to carbide tipped saws could afford any suggestion for the design of a carbide tipped saw capable of satisfactorily cutting non-ferrous industrial metals when driven at the low surface speeds at which a high-speed steel saw is normally operated for cutting such metals. Furthermore, the problem is one that is peculiar to the sawing of non-ferrous metals, and therefore no help could be expected from teachings relating to saws for ferrous metals.
As examples of patents disclosing prior carbide tipped saws for cutting industrial non-ferrous metals, intended to be operated at high surface speeds, reference can be made to U.S. Pat. No. 2,720,229 to Drake and U.S. Pat. No. 3,576,200 to Elmes. An early disclosure of a currer for non-ferrous metals that could take the form of a saw with high-speed steel teeth is U.S. Pat. No. 543,608 to Beale.